The present invention relates to a transmission circuit and a radio transmission apparatus in which there is a need to control the transmission output gain and, particularly to a digital transmission circuit and a digital radio transmission apparatus which are suitable for a mobile telephone and the like using a code division multiple access (CDMA) system.
The CDMA system is attracting attention as a signal modulation method suited for a large capacity radio transmission apparatus. In a radio transmission apparatus such as a mobile telephone using the CDMA system, the transmission output gain must be controlled in accordance with demands from a base station of a mobile telephone system.
FIG. 1 is a block diagram showing the constitution of a conventional transmission apparatus in a CDMA mobile telephone system.
An I-phase transmission data signal I-DATA and a Q-phase transmission data signal Q-DATA, which are orthogonal transmission signals output from a data processor (not shown in FIG. 1) and are spread-spectrum processed, are input to a modulator 51. The modulator 51 is an orthogonal modulator and modulates an intermediate frequency local oscillator signal output from a local oscillator 52, in accordance with the above-described orthogonal I-phase transmission data signal I-DATA and the Q-phase transmission data signal Q-DATA.
The modulator 51 supplies outputs to an IF-AGC amplifier (intermediate frequency automatic gain control amplifier) 53. In accordance with, for example, a gain adjustment demand signal (TX AGC ADJ1) from the base station, the IF-AGC amplifier 53 amplifies the modulated signal which has been input thereto at a gain determined by the gain adjustment demand signal TX AGC ADJ1 generated from the base station provided in the mobile telephone system. The IF-AGC amplifier 53 supplies outputs to an up-converter (frequency converter) 54.
A local oscillator signal generated from a TX-RF local oscillator 55 is input to the up-converter 54 which converts the frequency of the modulated signal in an intermediate frequency band to a signal in a transmission path frequency band.
A band-pass filter (BPF) 56 deletes unwanted frequency components from the modulated signal which has been converted to the transmission path frequency band to obtain an RF signal. Thereafter, the RF signal is input to a power control amplifier (PC-AMP) 57. The power control amplifier 57 amplifies the modulated signal input thereto to a gain determined by a gain adjustment signal TX AGC ADJ2 which is generated together with the gain adjustment signal TX AGC ADJ1. The power control amplifier 57 delivers outputs to a power amplifier (PA) 58. The power amplifier 58 amplifies the power of the modulated RF signal which is output from the power control amplifier 57. The RF signal which has been power-amplified by the power amplifier 58 is supplied to a transmission/reception antenna (not shown) for transmitting a corresponding electric wave into space.
In FIG. 1, the circuits including the modulator 51 and the IF-AGC amplifier 53 are integrated within the same integrated circuit.
In this conventional transmission apparatus, in attempting to control the transmission gain by using only the IF-AGC amplifier 53, when the power output from the power amplifier 58 is at its minimum, the up-converter 54 has a high noise figure (NF) of approximately 15 dB, and a power gain of approximately 10 dB. Consequently, the S/N ratio of the signal output from the up-converter 54 deteriorates, as does the waveform quality ρ (ratio of signal components to signal component+noise component :ρ=S/(S+N)). To solve such problems, in the circuit of FIG. 1 the gain of the power control amplifier 57 is made variable and is lowered in accordance with the gain adjustment signal TX AGC ADJ2.
Generally, the power control amplifier 57 has an NF of approximately 6 dB. Consequently, its gain is small and as long as the overall output gain is adjusted while reducing the noise of the up-converter 54, which is dominant in the transmission apparatus, it is possible to prevent the waveform quality from being deteriorated.
However, when the power control amplifier 57 is configured such that its gain can be controlled, the constitution becomes complex, increasing the cost of manufacturing the mobile telephone. Moreover, since the number of constituent parts increases, there is a problem that the external size of the constitution of the power control amplifier 57 becomes large.
Two control lines connected to the amplifiers 53 and 57 are needed for adjusting the respective gain. When attempting to use only one control line, an additional circuit becomes necessary, thereby further increasing the number of constituent parts and increasing the area of the IC circuit.
In addition, since the modulated signal which is amplified by the power control amplifier 57 has an RF high frequency and the output power is large, it is technically difficult to integrate the power control amplifier 57 with another IF-AGC amplifier 53 or the like to form a single integrated circuit.
This invention is made in consideration of the problems described above, and aims to provide a transmission circuit and a radio transmission apparatus which can be inexpensively manufactured without increasing their external sizes and are capable of obtaining a transmission output having a good waveform quality with low noise even when the gain is greatly adjusted.
It is another object of this invention to provide a frequency converter which can easily obtain a wide variable gain amplitude and can depress the NF when the gain is adjusted in the minimum range.